This invention relates to a method of and apparatus for growing crystal ribbons by the EFG (Edge-defined Film-fed Growth) technique. The EFG process has been used to grow ribbons of various crystalline materials from liquid melts. Ribbons so grown usually include microvoids formed from gas in the melt that becomes trapped in the ribbon as it forms. The presence of these microvoids is not always acceptable. For example, sapphire ribbon, grown from a liquid melt of aluminum oxide (Al.sub.2 O.sub.3) is cut into wafers and these wafers are used as substrates on which silicon is grown. The resulting silicon-on-sapphire (SOS) wafers are processed to include integrated electronic circuits. The presence of microvoids, particularly those of relatively large size, in the sapphire has a detrimental affect on the performance of the circuits. In order to reduce the number and size of the microvoids, the raw material that eventually forms the melt and the chamber in which the melt is formed have been subjected to various outgassing techniques to eliminate trapped gases. These techniques have helped, but have not eliminated or consistently minimized the presence of microvoids in the ribbons.